I had this a few weeks ago in a discussion thread about smash brothers. Let me just copy the actual comment over before summing it up:

Source (https://www.reddit.com/r/smashbros/comments/a0bbqr/would_you_want_a_super_smash_bros_ultimate/eah53tx/)
I'd want it to work like this:

FIGHTER DECK - this is a smallish deck of cards themed around a single fighter - link, fox, etc. It has cards which represent moves and items that they use. There's only one Final Smash card in the deck, and they have some restriction on use, but the exact restriction varies depending on the final smash (for example, maybe Marth's can only be played the turn after you've hit the enemy with one of your sword attacks, representing its short range, while Ganon's might require you to collect 3 Triforce cards before being usable).

You draw cards from your fighter deck and play them from your hand, and different characters have different balances of card types (offensive, defensive, etc) - look at games like Sentinels of the Multiverse or Red Dragon Inn for example of how this kind of character deck works. But that's not all, because there are 2 other kinds of decks.

The first is the Spirit deck. This is where the collectible nature of the game comes in - you have a Spirit deck made up of however many Spirit cards you want to be using (the rules will probably specify some minimum and maximum), and many cards in your fighter deck will refer to your spirit deck for various purposes. For example, you might have cards that just let you immediately play one spirit and see what its card does. But you might also have cards that let you burn a spirit card and make an attack which was modified by some aspect of the spirit card (for example, if each spirit has a "power" attribute on it, you might have an attack that burns a spirit to deal damage equal to it's power).

That's where the strategic aspect of the game would really be at its best, I think - choosing spirits that will synergize with the style of your fighter deck. Maybe Peach's deck is all about using and boosting spirits - you'll want your spirit deck to be full of effects that damage your opponent. On the other hand, if you're playing a deck that attacks a lot, your spirit deck might be more about defense.

Finally, the last deck type is the stage. Stages have a mix of positive, negative, and neutral effects. There will be fighters that work better in certain stages than others, of course. For example, a stage like, say... the Temple of Time... might have cards that make you go through your deck faster than normal, by forcing you to draw and play extra cards every turn. If you're a deck that likes burning through cards (let's say that Sonic has lots of cards called Sprints which have a minor effect, and a few cards called Marathons which work better with a lot of Sprints in the discard), you'll love playing there. After each player has had a turn, the stage plays a card automatically. Again, look to Sentinels of the Multiverse to see how this kind of thing works.

When you buy a fighter deck, you also get a random pack of spirits and a stage deck (that way, you're guaranteed to have a fighter to play with, spirits to use, and a stage to play on). You can even do a stage morph by alternately drawing cards from 2 stage decks.

I think the object of the game at its base is to deal X amount of damage to the other player, with characters having a varying amount of health (characters that deal lots of damage fast, like Sheik, have low levels of health, but characters that slowly build up to devastating combos, like Bowser, have lots of health. And of course you have weird things like Kirby has cards them let him eat a spirit to heal a little, but permanently remove it from the game). But there are probably ways to have alternate win conditions. Maybe Ganon's final smash card just wins the game for him, but he needs 3 Triforce pieces in his hand to play it (which is tough, since that's four cards you need all together, and you can't play them even though each Triforce has a nice buff...). Maybe there are boss spirits that have their own health and powerful attacks, and let you win if you defeat them (and you build your deck around playing and beating that spirit before your opponent does). Maybe you play with knockback rules, where you've got to deal a some amount of damage in a single turn to win.

It would be pretty cool, I think.

To sum it up: Basically you'd have a pre-built fighter deck, similar to games like Red Dragon Inn or Sentinels of the Multiverse, but beside that you also have a spirit deck, representing items, assist trophies, etc, which have a lot of possible effects. There would be cards in your fighter deck that either let you play from the spirit deck or burn spirit cards for their own effects, or a variety of other things. Optionally there's an stage deck that gets played automatically.

I've continued to think about this idea since pitching it in that thread, and I think it might actually work as a card game separate from smash. So let's just just file the numbers off and see what we get?

You have a pre-built character deck, which mostly determines how you'll be playing - it has some balance of cards that defend, attack, heal, etc, varying by character. Your hand comes from here. On your turn you get to play one card from your hand and then draw one card from your hand. Some of the cards from your character deck let you play from your assist deck, which instead of being in your hand just sits face-up - you can always see what's on top and thus will be played next, but you can't choose what it is. Your goal is to deal enough damage to your opponent to knock them out - different character decks have different amounts of health.

I'm gonna spit-ball varies ideas in different spoilers.

How to work in Final Smashes - or more generically, Ultimates? The original idea is that it's a unique card which you need to set up particular circumstances to play, and I think that's the right way to go - some characters can have Ultimates that are less powerful and easier to use, while others can have Ultimates which are very powerful and hard to use. I think they should always have some restriction though, even if it's a really simple one.

A big issue with balance for this hypothetical game is probably going to be the assist deck. How to balance a deck that can be custom-built? All the ideas I've had so far have seemed pretty ****ty.

You could have it be limited by card value. You have a maximum total value. The problem with this is that with a game that feeds your discard back into the draw, having a small deck tends to be more powerful - you can get through it faster. If you're choosing between "many weak cards" and "a few strong cards", having a few strong cards is going to be flat-out better, because you play them more often.

It would be very difficult to have all the cards be just as strong as each other, which would be the only way to have things be balanced with a static assist deck size. It would also be pretty boring.

One thought is that more powerful cards could have a cost or a limit of some sort. Maybe stronger cards, after being used, get burned instead of just going into the discard, so you can only play them once. I don't think I really want to introduce something like Magic's mana, and Yu-Gi-Oh's sacrificing cards to play stronger ones seems more suited to the gimmick of an individual character deck...

This is something to keep thinking about, I guess.

Every game needs something to tie it together thematically. I want this game to be very wide-ranging, not limited to just fantasy or just sci-fi, or anything so... something involving universes colliding, maybe? That way you can easily get in everything from knights to superheroes to starship captains.

Obviously there needs to be a very simple character for people to try. Their cards should be uncomplicated, and their interaction with the assist deck probably limited to "play the top card of your assist deck". This should probably also be the character that other characters are balanced against. The Mario of this game, basically. Their ultimate is probably a simple attacking card that doesn't have much restrictions on its use.

Thematically, maybe this is a soldier of some sort? Maybe a knight?

Another character that needs to exist is one that's mostly attacking and not much defense. Again, probably an uncomplicated character, but their interaction with the assist deck is probably much more "discard the top card of the assist deck and deal damage equal to its value", or something like that. Their ultimate is probably a very powerful attacking card - or maybe it's a retaliation thing, where after they take damage they can play it as well as another card from their hand to deal damage?

Thematically, this is probably a gunman of some sort. Maybe this is a space-fighter pilot?

Again, a simple character but a necessary one. Probably starting to approach complex though - I think it makes the most sense for them to have cards that come out and stay out, and have a small effect each turn - reduce damage by 1, deal 1 damage every turn, etc. Alternatively, maybe they have cards that they can play out-of-turn to reduce damage. Their ultimate is probably a defensive thing - maybe you play it when you would have taken damage to heal instead?

Thematically, how about this is a superhero? Protecting the innocent (and also yourself).

Here we are with a definite weirdo. This is gonna be a character that is majorly focused on the assist deck, and has cards for permanently burning up assist cards for powerful effects. This is Kirby, in other words. They're a little more fleshed-out in my head that the other concepts - I imagine that one of their important things is a card which lets them essentially bank to the top card of their assist deck, taking its powers and being able to use it repeatedly. The limit is probably that they can only start with a 1-value assist, and the next time they play it they can take a 2-power assist, and so on. Their ultimate is definitely a powerful effect that burns up whatever they're currently banking.

Thematically, this is probably some sort of eldritch being or maybe a possessing ghost.

Mechanically, I feel like there should also be...
A deck centered around damaging yourself in order to use your cards/spirits. Lots of health, some self-healing.
A deck centered around using and boosting the effects of your assists.
A deck centered around stealing your opponent's assists.
And more, probably.

Thematically, I feel like there should also be...
A wizard of some sort.
A demonic warlock of some sort (maybe this is the deck that damages itself?)
A thief of some sort (this is probably the deck that steals your opponent's assists)
A modern-day soldier.
A time-traveler.
And more...

Gun. It deals damage.
Sword. It deals damage.
Cleric. They heal you.
Wildfire. Deals damage to both players.
Ghost. Steals an opponent's card.
Magic Shield. Protects you from damage.
Magic Sword. Deals more damage when you deal damage.
And more...

Apocalyptic Future. Has a card that plays automatically when it reaches the top of the assist deck - after X number of turns, ends the game immediately and both players lose. If stopped by some effect before then, both players win (maybe better suited to being part of a stage?). Other cards involve time paradoxes and marauding leather-wearers and such.
Island Paradise. Generally positive for both players.
City Center. Has bystander cards that need to be protected from criminal cards maybe?
And more...